A coordinated program of physiological and morphological experiments is planned to acquire greater understanding of the events involved in the development and specification of synaptic connections. Electrophysiological experiments are performed on identified developing nerve-muscle junctions and followed by light and electron microscopy of the same nerve terminal. The pimary aims of the proposed experiments are: (1) to demonstrate systematically the maturational changes in the transmitter release properties that occur during the development of the amphibian nerve-muscle junction and to determine the chronology of these changes; (2) to determine the morphological changes of both the developing nerve terminal and the postsynaptic muscle fiber during ontogeny and to correlate these with the changes in the nerve terminal's physiology and (3) to determine what factors control the specificity of formation and elimination of synapses during neuromuscular development.